Invasion patterns vary depending on the scale. At finer scales, invasions are often related to competition. For example, kudzu sprawls across 227,000 acres of U.S. road edges, abandoned fields, and neglected areas. Kudzu out-competes other vegetation, eventually smothering native plants.

At broader scales – such as the southeastern U.S., or the state of Alabama – invasions are usually related to humans. Human travel, trade, and other activity can inadvertently spread plants to new areas. “Many species are overcoming natural dispersal barriers at unprecedented rates,” says Guo. “The result is increased homogenization of the earth’s biotas.”

“At the global scale, no species are exotic,” says Guo. “All species are native to the planet.” When very large areas are considered, the number of exotic species shrinks. The opposite is also true – small areas have much larger exotic pools.

This insight helps explain why islands sometimes seem more susceptible to invasion. Islands are typically small, which means their exotic pool is large.

In addition, some islands, such as Hawaii, also have many human residents and visitors, which translates to higher propagule pressure. Propagule pressure describes human’s roles in introducing new species.

Guo, a research ecologist at the SRS Eastern Threat Center, recently led a study comparing plant invasions across regional and global scales. The results were published in Landscape Ecology.

Guo and his colleagues compiled data for 100 regions, countries, or states, as well as 89 islands across the globe. The scientists compared plant invasions at regional and global scales. They also considered the effect of geography – were islands invaded more often than mainland areas?

The relationship between area and the proportion of exotic species held true across mainland areas and islands. “Therefore, there is a continuum in degree of invasion across mainland areas and islands,” says Guo.

At regional scales, invasions are often related to human activity. For example, mowers and other equipment can carry cogongrass seeds to new areas. Cogongrass is among the 10 worst weeds in the world. Photo by Steve Hillebrand USFWS.

“At a global scale, the proportion of exotics is partly determined by land area of concern,” says Guo. “Thus, islands may not be more susceptible to invaders than continents. Rather, due to their smaller size, islands may have more species defined as exotic and therefore have a larger exotic species pool.”

The study also showed that in areas with shared borders, human population density drives invasion patterns.

“At local and regional scales, the proportion of exotic species is likely to increase,” says Guo. “We expect invasions to continue until most species that could establish in new environments have done so, or until highly effective management is in place.”

Managers should consider the implications of travel and land use and be prepared for higher propagule pressure in areas where humans visit and live.